This invention relates generally to digital voice communication systems, including but not limited to radio frequency (RF) communication systems.
In recent years, digital communication systems have become an increasingly popular alternative to conventional analog communication systems. Either system typically includes a plurality of receivers, that may be mobile or portable communication units, base stations, or repeaters capable of receiving signals, such as voice signals. As an example of such a system, a first communication unit transmits a voice message via a radio frequency (RF) communication resource directly to a second communication unit. Alternatively, the first communication unit may transmit the voice message to a repeater which in turn retransmits the message to the second communication unit. Analog and digital communication systems fundamentally differ in the manner of transmitting and receiving the voice message and in the manner of accommodating errors in the voice message.
In analog systems, the original analog voice waveform is used to directly modulate an RF carrier, yielding a modulated carrier signal that is transmitted via a RF channel to a receiver. The receiver demodulates the signal to recover the original analog voice waveform, which may include audible noise resulting from channel impairments such as out of range or fringe conditions, or resulting from interference such as co-channel, adjacent channel or multi-path interference. Generally, users of analog systems are accustomed to hearing noise in the recovered signal indicative of the type of error conditions encountered during transmission. For example, error conditions resulting from the receiver approaching the fringe portions or outer limits of a coverage area usually result in weak recovered voice signals and increasing amounts of background noise, whereas error conditions resulting from interference subjects otherwise strong recovered voice signals to periodic interference from other signals. Thus, in an analog system, the user can readily distinguish between the types of channel impairments causing poor quality recovered voice signals. In digital systems, the analog voice waveform is digitized and then encoded prior to modulation and transmission over the RF channel by adding redundant bits that can be used for the detection and correction of errors. When channel impairments are encountered, errors in the received signal can be corrected at the receiver such that the original voice signal can be reconstructed with little or no interference. Generally, therefore, for moderate error conditions, the error correction capability of digital systems allows for transmitting voice messages with better audio quality than analog systems. Clearly, better audio quality is a desirable feature in many respects but it inherently prevents the user from hearing noise in the recovered signal that would otherwise be indicative of error conditions. For example, the user might hear a relatively clear voice signal in a digital system even if the receiver is approaching the fringe portions or outer limits of a coverage area. Moreover, the correction of errors in a digital system will cease when too many errors have occurred in the data. During such high error conditions, the receiver silences, giving the users no indication of what went wrong. The user may conclude during these periods of silence that the system is not working, when in fact the silence is due to channel impairments such as out-of-range or fringe conditions, co-channel or adjacent channel interference. The user has no way of determining what type of error condition is causing the silence. Moreover, when a repeater is used, the user has no way of determining whether the error condition occurred on the inbound- or outbound-side of the repeater.
Accordingly, there is a need for enhancing a digital communication system with a mechanism and/or method for indicating and differentiating between received error conditions. The system will preferably enable users of communication units to distinguish between errors due to out-of-range or fringe conditions and those due to interference conditions. Where a repeater is used, the system will preferably enable users to distinguish between errors occurring on the inbound- or outbound-side of the repeater. The present invention is directed to satisfying or at least partially satisfying the aforementioned needs.